1. Field of the Invention
The present invention relates to a wireless communication system, a wireless communication device, a wireless communication method and a computer program for data communication with transmission capacity increased by a space-multiplexing (MIMO) communication system between a pair of a transmitter and a receiver each having two or more antennae. More particularly, the invention relates to a wireless communication system, a wireless communication device, a wireless communication method and a computer program in which each of the transmitter and the receiver transmits and receives using a waveform equalization algorithm in one of a linear area and a non-linear area to attain the best link characteristic.
2. Description of the Related Art
Wireless networks are now attracting attention as novel communication systems to replace wired communication systems. Exemplary standards for wireless networks may include institute of Electrical and Electronics Engineers (IEEE) 802.11 and IEEE802.15. IEEE802.11a/g is a standard of wireless LANs employing orthogonal frequency division multiplexing (OFDM) modulation, which is a multi-carrier system.
Although the IEEE802.11a/g standard supports a modulating system which achieves a transmission speed of 54 Mbps at maximum, there is an increasing demand for a higher bit rate next-generation wireless LAN standard. A multi-input multi-output (MIMO) communication system is now attracting attention as a technique for providing high speed wireless communication. An OFDM_MIMO communication system is adopted in IEEE802.11n (TGn) which is an extended standard of IEEE802.11.
MIMO is a communication system which provides a space-multiplexed stream between a transmitter and a receiver each having plural antenna elements. The transmitter multiplexes plural transmission data by spatially and temporally encoding the data and transmits the data to channels via plural transmitting antennae. The receiver spatially and temporally decodes a signal received at plural receiving antennae via the channel to separate the signal into plural transmission data. In this manner, the original data can be obtained without causing crosstalk between the streams. The MIMO communication system can increase the transmission capacity in accordance with the number of antennae and improve the transmission speed without having to expand a frequency band. The MIMO communication system utilizes spatial multiplexing, which ensures greater frequency utilization efficiency. The MIMO communication system takes advantage of channel characteristics, and thus differs from a simple transmitting and receiving adaptive array.
In the MIMO communication system, a transmission weight matrix for spatially multiplexing transmission streams from plural transmitting branches and a receiving weight matrix for spatially separating the spatially-multiplexed signal are calculated by the receiver using the channel matrix H. The channel matrix H is a numerical matrix with element channel information corresponding to a pair of the transmitting and receiving antennae. The channel information herein refers to a transfer function which has phase and amplitude components. Usually, the channel matrix can be estimated by performing a frame exchange sequence including a training sequence having a known reference symbol for exciting the channel matrix between the transmitter and the receiver.
SVD-MIMO is known as one of systems which can provide the best link characteristic in the MIMO communication system. The system obtains a transmission beam-forming matrix V through singular value decomposition (SVD: Singular Value Decomposition) of the channel matrix H into UDVH (i.e., H=UDVH).
A relatively simple algorithm for obtaining a receiving weight matrix from the channel matrix H at the receiver which receives a beam-formed spatially-multiplexed signal is also proposed. Examples thereof include zero force (ZF) which simply uses an inverse matrix H-1 of the channel matrix H for the receiving weight matrix on the basis of the logic to remove crosstalk completely, and a minimum mean square error (MMSE) receiving system in which the receiving weight matrix W is calculated from the channel matrix H on the basis of a logic which maximizes a ratio of the electric power of the signal to a square error (i.e., the sum of crosstalk electric power and noise power). The ratio is also called SNR. MMSE is an algorithm which adopts a concept of noise power in the receiver and generates crosstalk intentionally to obtain the receiving weight matrix W. It is known that MMSE is advantageously used in an environment with a large amount of noise as compared to ZF.
Another algorithm for spatially separating the spatially-multiplexed signals is maximum likelihood detection (MLD) which estimates the maximum-likelihood transmission sequence by matching what with all the possible transmitted signal series patterns. Although MLD is known as a receiving system of excellent performance, it has a problem in that the operation scale is large and thus mounting is difficult.
A wireless communication system which improves the original receiving characteristic of SVD while reducing an operating load of SVD and MLD by combination of the SVD-MIMO communication system and the MLD reception system has been proposed (see for example, Japanese Unexamined Patent Application Publication No. 2007-110203).
Completely orthogonal channels are therefore formed between the transmission side and the reception side to provide the best link characteristic. The receiver can receive signals of any receiving system as long as the system is a linear area estimation system.
It has been found, however, by the inventors that in a case where the number of transmitting antennae of the transmitter is equal to the number of transmission streams formed by transmission beam-forming, when the same modulating systems are assigned to plural streams, the expected characteristic improvement effect of the MLD reception system which is the system for estimating the non-linear areas is hardly obtained and the characteristic becomes equivalent in either the MLD or the MMSE reception system with respect to the beam-forming transmission packet under some communication conditions (i.e., the condition of the channel matrix H).